The present invention relates to a glass substrate, which is applied to the flat panel display (citing FPD hereinafter) such as the liquid crystal device panel and to a manufacturing method thereof. The present invention is particularly useful for a glass substrate using for a large size FPD for example televisions, computer displays and the like.
In a few years, FPD represented by a liquid crystal type and a plasma type, spreads rapidly. An emission type display was also released recently, and a development is further activated.
For manufacturing these types of FPD, it is necessary to precisely form minute patterns on a tabular glass plate. For example, the explanation will be provided below in accordance with a TFT type liquid crystal device. The TFT type liquid crystal device is made by forming and arranging a plurality of thin film transistors on a glass substrate in precision to match each TFT on each pixel. For arranging TFT precisely a photolithography technology is used.
Namely, a metal layer is formed on a first glass substrate, and then photoresist is coated on the metal layer. Next, after TFT patterns for plurality of panels are exposed and developed, etching is performed. As the result, the metal layer remains in a shape of TFT patterns on the first glass substrate. This layer is typically referred to as the “TFT substrate.”
On a second glass substrate, a shading material layer is formed and further photoresist is coated on the shading material layer. Next, a plurality of color filter (citing CF hereinafter) patterns, which will correspond to TFT patterns, are exposed and then etched. As the result, the shading material layer remains in a shape of CF patterns on the second glass substrate. Next, by using photolithography technology, which is the same as that used for forming the TFT patterns, CF is formed in accordance with the patterns of the shading material layer. A red filter, a green filter, and a blue filter are formed by repeating CF forming process three times. This layer is typically referred to as the “CF substrate.”
After an alignment layer is coated on each of the TFT substrate and CF substrate, both substrates are adhered together through glass beads as spacers in a state that the alignment layer of each substrate becomes an inner side, and further, out of pattern areas are adhered by sealant.
Next, after cutting out each panel, material of liquid crystal is injected to a space between the TFT substrate and the CF substrate through a hole previously provided for supplying the material of liquid crystal, and the hole is sealed. Finally, a polarizer is adhered on a screen, and then the TFT type liquid crystal panel is completed.
Each pixel of TFT pattern should be aligned with each area of correspondent CF pattern. If the patterns are misaligned with respect to each other, a precise image cannot be processed. Therefore, both the TFT pattern and the CF pattern should be formed in high dimensional precision.
As an index for estimating whether the TFT pattern or the CF pattern is formed in predetermined dimensional precision, a plurality of measurement patterns are provided out of the TFT pattern or the CF pattern. After exposing the TFT pattern or the CF pattern and then developing, distances between these measurement patterns are measured and the difference from design values are obtained, and then preciseness is estimated.
In the meantime, a majority of glass substrates used for FPD are manufactured by a process called fusion process. The fusion process is the method for manufacturing a plate like glass involving flowing a fused glass into a container called a fusion pipe, overflowing the fused glass from the fusion pipe, and solidifying the fused glass during downward flow of the fused glass. The fusion process can manufacture glass substrates in a low cost because a polishing process is not needed.
A method for manufacturing a glass substrate by fusion process will be explained below. FIG. 8 shows a fusion pipe which is used for manufacturing a glass substrate by fusion process. In FIG. 8(a), a fusion pipe 4 has a structure that its upper part is a trough like portion 41 which is open to upper direction, and that both sides of the trough like portion 41 are bank like portions 42 whose levels are higher than the that of the trough like portion 41. A lower part of the fusion pipe 4 has a wedge like shape, and its bottom is a blade like portion 43. Heaters (not shown) are built in the fusion pipe 4 and surfaces of the fusion pipe 4 can hold a temperature at which maintaining a glass-fusing state. A cross sectional view of the fusion pipe 4 in A-A direction is shown in FIG. 8(b).
FIG. 9 shows a process for manufacturing a glass substrate by the fusion process. A fused glass G flows successively into the trough like portion 41 of the fusion pipe 4, which is maintained at a high temperature. The fused glass G overflows from bank like portions 42 to both sides of the fusion pipe, further flowing down along side surfaces of the fusion pipe 4, and reaches the blade like portion 43, which is at the bottom of the fusion pipe 4. At the blade like portion 43, flows of fused glass join together and forming plate like glass GP, and the plate like glass GP is gradually cooled as it flows downward. In this process, the plate like glass GP is gradually solidified, and further it is pulled down by a rotation of rollers 45. Afterwards, by cutting in desired dimension a glass substrate is completed.
Additional manufacturing processes involving the fusion process are described in following documents:
U.S. Pat. No. 3,338,696
U.S. Pat. No. 3,682,609
To enlarge FPD and to increase efficiency of manufacturing FPD, a size of a glass substrate for FPD is expanding year by year. As far as a substrate for liquid crystal is concerned, a size of the substrate called seventh generation, which is already practically used, is very large as 1870 mm×2200 mm, and still larger size substrate is proposed.
However, in forming patterns on a large size glass substrate, there is a problem that the dimension of the actually formed pattern occasionally has an error beyond allowable range in comparison with design value.
As far as this problem is concerned, as optical systems of an exposure apparatus, which is used for exposing patterns, is adjusted in high precision level, it is confirmed that patterns to be exposed have substantially no distortion. Namely, it is not due to a precision level of optical systems. Moreover, a stage of the exposure apparatus is finished as a very high precision flat surface, and it is confirmed that the surface of the vacuum contacted glass substrate on the stage is in the focus depth of the exposure optical system installed in the exposure apparatus. Therefore, it also does not due to a problem of flatness level of the stage. Accordingly, it can be considered that distances between measurement patterns exposed on the glass substrate are in allowable ranges at least in the state that the substrate is vacuum contacted on the stage of the exposure apparatus.
However, in spite of above confirmations, still there is a problem that the distances between measurement patterns which are measured after exposing and developing are occasionally not in the allowable ranges.
Moreover, as another problem; there is a phenomenon that a static electricity is generated when the glass substrate, being coated with photoresist and being exposed the TFT patterns or CF patterns, is unloaded from the stage; and by discharging the static electricity to a surface of the photoresist, a defect of the TFT pattern or the CF pattern occurs.
The inventors of the present invention have found out that there is an effect for these problems by decreasing the contact area between a vacuum contact portion of the stage and the glass substrate. For decreasing the contact area between a vacuum contact portion of the stage and the glass substrate, the present inventors formed an asperity on at least one of the surfaces of the vacuum contact portion of the stage and the glass substrate. The present inventors formed minute asperity on the surface of the vacuum contact portion of the stage by a grinding process. Despite that an effect for preventing the static electricity is achieved in a short period after the grinding process, the effect decreases as time passes, and it also appears that the effect increases again by applying the grinding process again on the surface of the vacuum contact portion of the stage. One reason why the asperity on the surface of the vacuum contact portion shrinks during successive contacts with the glass substrates may be due to attrition with the glass substrate. As a result, the effect for preventing the static electricity decreases.
Although it is possible to form an asperity by machining the glass substrate after it is manufactured, such a process increases the cost for manufacturing the glass substrate. In order to decrease the cost, it is desirable to establish a method for manufacturing the glass substrate in which an appropriate asperity is formed in process of making the substrate.
The various embodiments of the present invention were made under the circumstances explained above. Objects of the present invention, include, but are not limited to, providing a glass substrate which enables the manufacture of a FPD low cost, and providing an efficient manufacturing method of a glass substrate.